Crystal Structure of the High-Pressure Phase Silicon VI

High-pressure phases of group-IV, III–V, and II–VI compounds

“…Two other widely used schemes are the full-potential linearized muffin-tin orbital method (FP-LMTO; see Dreysse, 2000) and the fullpotential linearized augmented plane-wave method (FP-LAPW; see Singh, 1994 (Olijnyk et al) but its structure (Cmca) has only recently been resolved . Both LMTO calculations (Ahuja et al, 1999;Christensen, Novikov, and Methfessel, 1999) and PWPP calculations (Mujica et al, 2001a(Mujica et al, , 2001b confirm the exis-tence of an interval of stability for the Cmca phase with sh/Cmca and Cmca/hcp coexistence pressures that agree very well both among the different calculations ["33(2),41(5)… GPa (Christensen, Novikov, and Methfessel 1999); (34.0,41.5) GPa (Ahuja et al, 1999); (36.0,43.5) GPa (Mujica et al, 2001a(Mujica et al, , 2001b] and with the experimental values for the onset of the sh→Cmca transition (ϳ38 GPa) and Cmca→hcp transition (ϳ42 GPa; Hanfland et al, 1999). This agreement is all the more outstanding if one realizes that an error of ϳ20 meV per atom in the calculation of the difference in the energies of the phases would result in the total disappearance of the calculated interval of stability of the Cmca phase (see Fig.…”

“…On further increase of pressure the Cmca structure undergoes a transition to the hexagonal-close-packed (hcp) structure, which in Si starts experimentally at about 41 GPa Hu et al, 1986;Duclos et al, 1990;Hanfland et al, 1999) and in Ge at the much higher pressure of 160-180 GPa (Takemura et al, 2000). The hcp phase of Si undergoes a further transition to the face-centered-cubic (fcc) structure at a pressure of about 87 GPa, which remains stable up to the highest pressure reached so far, of 248 GPa (Duclos et al, 1990).…”

“…The recently discovered base-centered-orthorhombic Cmca structure (see Schwarz et al, 1998;Hanfland et al, 1999;Mujica et al, 2001a) appears in Si and Ge at high pressures as an intermediate between eightfoldcoordinated forms and 12-fold-coordinated close-packed structures. In Cmca the 8(f) sites are contained in flat (100) planes, while the 8(d) sites form corrugated layers that alternate with the 8(f) planes (see Fig.…”

Suppressing H2Evolution by Silicon Powder Dispersions

“…The theoretical amount of hydrogen gas evolved was calculated considering the face centred cubic unit cell structure of silicon crystals [19]. It was assumed that the Si was initially not oxidized and that exactly one layer of Si reacts to form SiO 2 .…”

Metallic high-pressure modifications of main group elements

“…The hp phase is the stable arrangement of Si-V (CN 8) at pressures above 16 GPa. Around 40 GPa, silicon transforms into the orthorhombic arrangement of Si-VI (CN 10 and CN 11) which is isotypic to Cs-V [54]. The atomic arrangement (Fig.…”